1. Field of the Invention
The present invention relates to an image reading apparatus, and in particular, to an image reading apparatus capable of obtaining both visible-light image information and infrared-light image information of an original. The image reading apparatus can be applied to, for example, an image scanner, a digital copying machine, or a facsimile machine.
2. Description of the Related Art
An image reading apparatus converts image information of a transmissive-type original (transmission original) into an electric signal to be loaded into a computer. Various proposals have been made on an image reading apparatus which obtains information on foreign matters and flaws on an original using infrared light and corrects the corresponding portion of visible-light image information on the original.
In U.S. Patent Application Publication No. US 2003/0132384 A1 and U.S. Pat. No. 6,493,061, an existent monolithic three-line sensor (a three-line CCD), an infrared light source, a visible light source, an imaging optical system, a focus adjustment unit, and a reading unit are used to obtain visible-light image information and infrared-light image information by performing sub-scan (reading in the sub-scanning direction) two times. Hereinafter, this scanning method is referred to as a “two-times sub-scanning method”.
In addition, an image reading apparatus in which LED (light-emitting diode) light sources are switched sequentially at each main scan (a reading in the main scanning direction) and visible-light image information and infrared-light image information are obtained by conducting a sub-scan once is proposed. Hereinafter, this scanning method using LED light sources is referred to as a “one-time scanning method by sequentially switching LED”.
According to the above image reading apparatus, both visible-light image information and infrared-light image information can be obtained in one sub-scanning by using an existent one-line sensor (one-line CCD), four light sources that emit light of four different wavelengths corresponding to red (R), green (G), blue (B), and infrared (IR), and an imaging optical system. The one-time scanning method by sequentially switching LED is advantageous over the two-times sub-scanning method in that the read time, component precision, and adjustment accuracy can be improved.
Furthermore, an image forming apparatus which obtains both visible-light image information and infrared-light image information of an original in one sub-scanning by a method of reading the image information by light of wavelengths corresponding to R, G, B, and IR simultaneously, using a specific, monolithic four-line sensor (four-line CCD), is discussed (refer to U.S. Pat. No. 6,590,679). Hereinafter, this scanning method using a specific four-line CCD will be referred to as a “one-time sub-scanning method using a specific CCD”.
FIG. 7 is a schematic diagram of essential portions of an image forming apparatus employing the one-time sub-scanning method using a specific CCD discussed in U.S. Pat. No. 6,590,679. According to U.S. Pat. No. 6,590,679, image information by light of wavelengths corresponding to R, G, B, and IR can be received simultaneously using a specific four-line CCD. Moreover, filter lines 608, 610, and 612 placed in the vicinity of a four-line CCD block infrared light from reaching R, G, and B sensor lines 602, 604, and 606, and block visible light from reaching an IR sensor line 614. At the same time, the chromatic aberration by infrared light is corrected by employing a filter with a specific configuration, and image information by light of wavelengths corresponding to R, G, B, and IR together is formed on the surface of the four-line CCD. As a result, both visible-light image information and infrared-light image information can be obtained in one sub-scan. Thus, the one-time sub-scanning method using a specific CCD is advantageous over the two-times sub-scanning method in that the read time, component precision, and adjustment accuracy can be improved.
In addition, by employing a lighting system which has a simple configuration of only continuously turning on a light source which emits visual light and infrared light, the lighting system can be manufactured more easily (at a lower cost) compared to the one-time sub-scanning method by sequentially switching LED. Furthermore, U.S. Pat. No. 6,590,679 discloses an embodiment in which a light bulb is used. A representative example of a light bulb that is used in an image reading apparatus is a halogen lamp. The halogen lamp emits large amounts of both infrared light and visible light, and can be easily manufactured (the cost is comparatively low). Moreover, the reference has many advantages such as conducting chromatic aberration correction by a filter line in the vicinity of a sensor so that an image reading optical system can be easily designed.
In FIG. 7, a light flux 634 comes from an original (not shown). The light flux 634 includes visible light 636 and infrared light 638. A second prism 640 transmits the visible light 636 and reflects the infrared light 638. A first prism 630 transmits the visible light 636. The infrared light 638 is reflected by a reflection surface 642 of the second prism 640.
In U.S. Patent Application Publication No. US 2003/0132384 A1 and U.S. Pat. No. 6,493,061 that employ the two-times sub-scanning method, since there is a backlash of the driving device which drives the carriage in the sub-scanning direction and deterioration over time occurs due to the thermal expansion of the whole system, it is necessary to restore the initial state of the optical system and perform an accurate positioning after the first sub-scan is finished. Therefore, the configuration becomes complex in order to improve the read time, component precision, and adjustment accuracy. Furthermore, by using a chromatic aberration correction unit (focus position adjustment unit), the structure becomes complicated (increasing the cost), and the entire apparatus becomes larger.
For an image reading apparatus using the one-time sub-scanning method by sequentially switching LED to read image information by light of wavelengths corresponding to R, G, and B, it is necessary to conduct reading in the main scanning direction by sequential switching. Therefore, a period of time required for reading visible-light image information becomes longer than that in the two-times sub-scanning method in which image information by light corresponding to wavelengths of R, G, and B is read simultaneously. In addition, the luminous efficiency of a LED light source is lower than that of other light sources such as a fluorescent tube. Therefore, in an image reading apparatus which reads a plurality of lines of film simultaneously, the lighting range becomes wide and the luminance becomes insufficient. Therefore, it becomes necessary to increase the number of LEDs to be installed. However, LEDs, in particular, blue LEDs, are difficult to manufacture (and therefore costly). Thus, using a large number of LEDs greatly increases production cost.
Moreover, for example, Japanese Patent Application Laid-Open No. 10-325921 discusses an imaging optical system in which the chromatic aberration is corrected by optimizing glass material, to form both image information by light of wavelengths corresponding to R, G, and B and infrared-light image information on the surface of a one-line CCD. However, the design of an imaging optical system capable of reading a plurality of lines of film simultaneously is difficult, and the number of lens elements greatly increases.
Therefore, an image reading apparatus employing the one-time sub-scanning method by sequentially switching LED is adequate to be applied to a film scanner for 35 mm film, because an image reading optical system and a light source do not become a heavy load. However, the manufacturing is difficult and the configuration becomes complex for applying to an apparatus which reads a wider width. In addition, in the above image reading apparatus, the number of times of sub-scan to obtain visible-light image information and infrared-light image information of an original is one. However, since image information by light of wavelengths corresponding to R, G, and B cannot be read simultaneously, the entire read time can not be considered short.
The manufacturing of a specific four-line CCD and filter lines discussed in U.S. Pat. No. 6,590,679 which employs the one-time sub scanning method using a specific CCD has the following problems. First, as for the filter lines, it is difficult to completely block visible light and infrared light, and there will be some leaking light. On the other hand, image reading apparatuses in recent years have a high gradation specification of approximately 8 bits, and even if it is considered that a noise component of 1 bit is allowable in general, the rate of leaking light is required to be less than or equal to ½(8-1)=0.8%. However, a filter line which meets this requirement is not easy to manufacture. For example, a specific dielectric multilayer structure as discussed in U.S. Pat. No. 6,462,866 may be incorporated in the filter line. Nevertheless, the manufacturing is still difficult.
In the case where the chromatic aberration correction is conducted by a filter line, considering that the amount of color deviation within a glass medium is generally 0.7 to 1.0 mm, the R, G, B sensor lines and the infrared sensor line need to be placed apart by that distance. Since the effective width of a conventional three-line sensor composed of R, G, and B sensors is approximately 0.5 mm, the device area greatly increases. Since the production yield of a device in a semiconductor process depends greatly on the area of the device, it can be foreseen that the cost will also increase greatly. Furthermore, there are also many issues by which a decrease in the yield can be expected, such as positioning the edge of the dielectric multilayer structure at an angle within a space of 0.7 to 1.0 mm, and aligning accurately with the CCD surface.
Moreover, an image reading apparatus employing the one-time sub-scanning method using a specific CCD can obtain visible-light image information and infrared-light image information of an original in one sub-scan, use a conventional reading optical system, and use a simple (low-cost) light source such as a halogen lamp. However, the manufacturing of a specific CCD and filter lines associated therewith becomes difficult.